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I have seen and heard sentences like these:

He could have gone too far.

John can't have eaten all the cake.

But I don't seem to encounter this structure:

She can have done the work.

I tried Googling it and found some examples, but I'm still unsure if it's acceptable.

I understand that "may have verb-ed" and "might have verb-ed" are frequently used in English. So if, in contrast, "can have verb-ed" is never used or is considered awkward, what's the reason for this?

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4 Answers 4

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An excellent question, which merits more investigation than might be possible here. The short answer is that in contexts like this can’t is not the negative of can, but the negative of must when used for deduction, as in She can’t have done the work and John can’t have eaten all the cake. The main uses of can in its positive form are to express possibility and ability, but not deduction.

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Oh, deduction. That's a hint. Thanks! –  KSS Apr 10 '13 at 9:06
    
+1 Barrie... but what about the incredulous version of "John CAN'T have eaten all the cake!"? How does that fit? –  John M. Landsberg Apr 10 '13 at 9:08
    
@John M. Landsberg. There it expressess possibility. Well, actually, impossibility in this case, because it's negative, and perceived or imagined impossibility at that. –  Barrie England Apr 10 '13 at 9:24
    
@JohnM.Landsberg It's sort of there in the absence ... Incredulous "John CAN'T have eaten all the cake!" Indignant reply "Yes he can! (have eaten all the cake)", meaning yes he did the greedy **7@^5$£" –  Mynamite Apr 13 '13 at 18:03

We do have a tendency to make this a conditional statement, thus: "She could have done the work." Although "she can have" is grammatically reasonable and valid, my feeling is that it is little used because the definitive nature of "can" seems to contrast with the conditional nature of the situation itself.

Let me try to elucidate: Merely saying she "can have done" something implies that she might not have done it. There is an embedded question: If we are not saying she "did" in fact do the thing, then why aren't we saying it in this definitive way? If we are saying that she "can" have done it ("she has the ability to have done it"), but we are not actually saying that she did it, then we start to wonder: Did she do it or not? We must at least be implying that she might not have done it. And that also implies that there might be some reason why she didn't do it, something that prevented her from doing it.

All of this uncertainty tends to lead us into a conditional mode of saying what we were trying to say in the first place, and we wind up saying "she could have done" the thing.

This is my best guess for why "can have done" isn't really favored in common usage, but as Bill Franke points out, it does occur, and it's not wrong.

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Now I'm understanding that 'can' is primarily about one's ability to do something and not used as a marker for uncertainty about the past, and this is up to 'could'. –  KSS Apr 10 '13 at 9:04

Google Ngram Viewer shows lots of "can have done" usages over the past 200 years.

The structure is grammatical, but it's like the USA's $1,000 bill: too difficult to use when all you're buying is a pack of chewing gum.

There are many complex tense+aspect structures, especially with modals, that no one uses anymore. But this isn't strange, because so many native Anglophones have discarded anything that's difficult for anything that's easy plus a hope and a prayer that everyone else has done the same and so will understand what they mean regardless of what they say.

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So you mean it's ok in grammatical terms but a dead usage? Then is it good to understand it's like the case of 'shall' and 'whom', which are almost out of use? My curiosity is downcast because of no reason for that! –  KSS Apr 10 '13 at 8:44
    
@KSS: Language changes over time. Some changes are good & some aren't. English has tended to become grammatically easier to deal with because the sequence of tenses & all the tenses & aspects are hangovers from the days when English was closer to German, which has complicated & difficult grammar, & when only the educated read & wrote English. Now that "everyone is literate", the midpoint of the curve has fallen to represent the lowest common denominator, which it did when most native Anglophones were illiterate & used oral language only. It's not a dead usage, though, just a rare one. –  user21497 Apr 10 '13 at 9:42
    
How many are different usages, such as I can have done the bathroom by 11 o'clock / I wonder how someone so young can have done so much (embedded question?) / procedures you can have done on your upper lid (reduced that-clause)? –  Edwin Ashworth Apr 10 '13 at 10:03
    
@Edwin: My second link shows some of the relevant usages. I don't have time to do that kind of research, I'm sorry to say, but I did verify that at least some of them were what the OP asked about & not simply a string that fit the words in the query. –  user21497 Apr 10 '13 at 10:20
    
@Bill: Thank you for theadditional information. I'll forgive your fit where we real native Anglophones would use fitted. Not that I ever learned to play the Anglophone. –  Edwin Ashworth Apr 10 '13 at 22:23

Could is (used as) the past tense of can, as well as an alternative to it in the present tense.

Thus, in speaking about what one might have been able to do in the past, as in your examples, could is generally the preferred verb.

This is true even when the verbs are negated; this Google Ngram shows that "could not have done" has been more frequently used than "can not have done" for at least the past 300 years.

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I also checked out the result of Google ngram. I should memorize the two structures. Maybe the simplest answer to my question is that it's the way English works! XO –  KSS Apr 10 '13 at 8:50
    
Your comment made me chuckle. It's amazing how often we're all tempted to say exactly that: That's just the way it works! :) –  John M. Landsberg Apr 10 '13 at 9:05

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